Rotational Spectroscopic and Ab Initio Investigation of the Rotamer Geometries of 2-Fluoroanisole and 3-Fluoroanisole

Abstract

The rotational spectra of 2-fluoroanisole (2-FA) and 3-fluoroanisole (3-FA) were investigated using Fourier transform microwave (FTMW) spectroscopy in the 4-26 GHz range. Assigned transitions correspond to the lowest energy rotamer for 2-FA which has the O-CH3 group directed away (anti) from the fluorine substituent whereas for 3-FA, the spectrum is consistent with the presence of two rotamers arising from syn and anti orientations of the methoxy moiety relative to fluorine. Ab initio calculations at the MP2/cc-pVTZ level were used to estimate the equilibrium (re) geometries of the three observed rotamers. Their assignments were confirmed through the observation of the rotational transitions of eight minor isotopologues ( 13 C and 18 O) in natural abundance for each species. The mass dependence (rm (1) ) structures derived using the experimentally determined rotational constants compare favourably with the ab initio estimates.The resulting sets of geometric parameters suggest that the aromatic ring backbone is distorted by the introduction of the angular methoxy substituent, with a tendency to induce bond length alternation around the ring, and by the electron withdrawing effects of fluorine.

Analysis of the Complex Quadrupole Hyperfine Patterns for Two Chlorine Nuclei in the Rotational Spectrum of 2,5-Dichlorothiophene

“…Similar structural effects are noted at C 2 in 2CT in comparison to the parent compound. The increase in the ring angle at the site of chlorination is smaller than that has been reported for fluorination on aromatic rings (2–3°). − This was previously explained in terms of the polarity of the C–X bond as a result of the inclusion on an electron-withdrawing substituent (X). This change results in more p-character in the hybrid orbital centered on the carbon directed toward the halogen atom.…”

Fluorination effects probed in 4-fluoroacetophenone and its monohydrate

“…Similar fluorination effect has been observed in previous rotational spectroscopic studies on the monofluorinated benzenes [44][45][46] cyanobenzenes, 47,48 phenols 49 and anisole. 50 For instance, in 2-fluoroanisole and 3-fluoroanisole, the increase (relative to the nonfluorinated parent species) in the C-C-C angle by 3.61 and 2.81 at the site of fluorine substitution was observed, respectively. 50 Similarly, in the 2-, 47 3-47 and 4-fluorobenzonitrile, 47 the largest geometric changes occur near the site of fluorination, resulting in an increase in the ring angle of B31.…”

“…50 For instance, in 2-fluoroanisole and 3-fluoroanisole, the increase (relative to the nonfluorinated parent species) in the C-C-C angle by 3.61 and 2.81 at the site of fluorine substitution was observed, respectively. 50 Similarly, in the 2-, 47 3-47 and 4-fluorobenzonitrile, 47 the largest geometric changes occur near the site of fluorination, resulting in an increase in the ring angle of B31. These rotational studies suggest that the most of significant geometric changes upon monofluorination occur in proximity to the substitution site, which can be attributed to the inductive effects arising from the electronegative of fluorine.…”